Cathode ray tube printer



July 1 2 L. E. MULOCK ETAL 3,041,947

CATHODE RAY TUBE PRINTER 3 Sheets-Sheet 2 Filed NOV. 16, 1959 July 3, '1962 L. E. MULOCK ETAL CATHODE RAY TUBE PRINTER 3 sheets-sheet 3 Filed Nov. 16, 1959 m 0E P 2 L 2 5:35: E2: 55 E mi 125% \E: :20 %?55 @2202: J a 55 3 I i J 02 a 5% 3 E58 55 53 z N 0E Wi wa F C1 tes Patent Bfiilfii? Patented July 3, 1962 3,041,947 CATHODE RAY TUBE PRINTER Lawrence E. Mulock, Endicott, N.Y., and Milton P.

Albert, University City, Mo., assignors to International Business Machines Corporation, New York, N.Y., a corporation of New York Filed Nov. 16, 1959, Ser. No. 853,187 Claims. (CI. 95-45) This invention relates to a cathode ray tube printer and more particularly to an arrangement for providing an automatic systems check of the printer.

The present invention is directed to a printer system which functions to transform information from magnetic tape into a character display on the face of a cathode ray tube. This display is then photographed on 35 millimeter microfilm to make a permanent record of the displayed information. In a high speed printer of this nature, it is extremely desirable to be able to automatically check, at various intervals during the operation of the system, the operation of component circuits to assure that they are functioning properly. Of particular interest in this regard are the selection and deflection circuits and the beam unblank control circuit. tIt is also advantageous to be able to check for drift of power supplies for the analog circuits and the digital circuits and for phosphor deterioration on the tube face.

In carrying out the above checking feature, the present invention makes use of a novel limit check arrangement which exercises the selection and deflection circuitry of the printer at their extreme limits of operation. During each film advance cycle, the deflection circuits are energized to cause the beam to deflect to a position below the right edge of the normal print area on the screen and to a position above the left edge of the normal print area on the screen so that all deflection control circuits will be checked. The selection circuits are also energized for proper selection of two characters which have complementary codes so that all selection control circuits are checked. As a result, there is successively unblanked in both the right bottom corner and the left upper corner of the screen a blank code square as would occur with the beam aligned with an opaque square on the character matrix so that no light reaches the screen and a retrieval code square as would occur with the beam aligned with a transparent square on the character matrix so that maximum light reaches the screen. A photo cell is located opposite each corner to view the display to determine if the above operations are satisfactorily completed. In the event that any of the above operations fail, a limit check interlock trigger will be turned on to indicate that the printer is not functioning properly.

Accordingly, the main object of the present invention is to provide an improved cathode ray tube printer system which is provided with an arrangement for automatically checking the operation of the system.

A further object of the present invention is to provide a cathode ray tube printer system which functions to transform digital input information into a'character display on the face of the tube and then to photograph the display on film, the system being further provided with means for checking the operation of the system once for each frame of data to be recorded.

A still further object of the present invention is to provide a cathode ray tube printer system having means for automatically checking the selection and deflection circuits, the digital and analog circuits, and the unblank control circuit once for each frame of data to be recorded.

A further object of the present invention is to provide a cathode ray tube printer system wherein the digital and analog circuitry and the selection and deflection circuitry are exercised to the minimum and maximum limit of their operation once for each frame of datato be recorded to check for proper functioning of the system.

A still further object of the present invention is to provide a cathode ray tube printer system wherein, once for each frame of data to be recorded, there is unblanked in each of two diametrically opposing corners of the screen two characters having complementary codes which are scanned by photo cell means to determine if the system is functioning properly.

The foregoing an dother objects, features and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention, as illustrated in the accompanying drawings.

In the drawings:

FIG. 1a is a block diagram showing the general arrangement of the system embodying the invention and including part of the logic circuitry for the limit check.

FIG. 1b is a diagram of the remaining logic circuitry for the limit check feature as applied to the system shown in FIG. 1a.

FIG. 2 is a diagrammatic view of the tube screen showing the unblanked limit check squares.

FIG. 3 is a timing chart.

Referring to FIG. 1a, there is shown the general arrangement of the printer system for transforming information from magnetic tape into a character display on the face of a cathode ray tube. The tape reader 10 which scans the digital data recorded on a magnetic tape may be any convenient type of reader, such as, for example, the well-known IBM type 729 Tape Reader. In the present instance, the information read from the tape is in a binary seven bit code pattern with each alphanumeric character being expressed in the CBA 8421 code. For example, the character A would be recorded on the tape and read as 1110001. From the tape reader the input information is passed to any suitable type of tape control unit or synchronizer 11 which acts as a buffer storage between the tape unit and the printer. The control unit shapes and stores the information bits from the tape and interprets controls between the printer and tape unit, such as supplying the necessary delays for tapes to be reversed, stopped, etc.

From the tape control unit the information is gated into a seven bit character register 12 and from the character register, it is gated under control of a suitable distributor ring 13 and timing control unit 14 into a selection register 15 and the horizontal and vertical positioning registers 16. In a normal print operation, a print control character will follow a signal character to set up to initiate printing. The next two characters will determine the horizontal position of the first printed character, the next two characters will determine the vertical position, and the fifth character, through the selection register, will determine the alphanumeric character to be printed. Each subsequent character will print an alphanumeric character, the horizontal register advancing one character posit-ion per character after the first alphanumeric character has been positioned. Subsequent characters will be printed one per cycle, each character being automatically positioned one space to the right of the previous one, and printing will continue in this fashion until the end of a line is signalled by a record mark character. The character following a record mark is recognized as a control character which selects the next line to be printed.

The digital information in the selection register and in the horizontal and vertical positioning registers is directed to respective digital-to-analog converters 17 and 18 which convert the parallel digital information to a corresponding current for use in the selection deflection yoke 19 and the positioning deflection yoke 20 of the cathode ray tube 21. The system preferably makes use of digital-toanalog circuitry, such as is fully disclosed in US. Patent No. 2,810,860, to which reference may be had for complete details of operation.

The cathode ray tube 21 is preferably of the image forming type wherein a tungsten lamp 22 and condenser lens assembly 23 are used to illuminate an external matrix array of symbols 24. The arrangement of the characters in the make-up of the letter chart on the matrix follows in this case from the standard teletype code and the use of binary stepped increments of horizontal and vertical deflection currents. The matrix is reduced by the lens 25 and focussed on the cathode ray tube photosensitive cathode 26 where the light images become current images. The photoelectrons liberated from the cathode when the matrix letter chart is projected on it are accelerated and focussed upon the plane of a selecting aperture 27. The aperture size is such that one letter only from the array can pass through it at a time. The magnetic deflection yoke coils 19 deflect the entire electron-image stream for letter-by-letter selection. The single-letter portion of the electron stream emerging from the aperture enters the positioning and reproducing end of the tube. Here it passes axially through a metal cylinder 28, the potential of which is maintained a few volts negative with respect to the aperture in order to suppress secondary emission. The cylinder 28 is under control of an unblank control circuit 29, to be described, and an unblank amplifier 30 to selectively pass the letter beam on to the high potential region where the positioning defiection yoke coils 20 focus and position the letter on the aluminized phosphor screen 31. The reproduced letters may be displayed in lines and columns as desired.

The data on the tube screen is photographically recorded on 35 millimeter film by a conventional recording camera 32. The system preferably makes use of a recording camera, such as is fully disclosed in Manual No. 203, dated July 1, 1955, and published by the Instrumentation Branch, Edwards A.F.B., California, to which reference may be had for complete details of operation. It will suifice to say here that a drive motor M is connected through a magnetic clutch-brake to a gear train which is the basic drive mechanism of the camera. This gear train, in turn, drives the shutter, in-out feed sprockets, the intermittent or frame positioning sprocket, and the synchronizing cams. On continuous or cine operation, film may be fed at the rate of 16 frames per second. The system may be programmed to give a film advance at the end of a printed line or at the first end-of-record mark in that line. For example, referring to FIG. 1a, when an end-of-record mark is encountered on the tape reader, a signal is emitted from the tape control 'unit 11 to set an end-of-record trigger 33. The positive output from the trigger 33 is switched at T clock cycle time, to be later described, by an AND switch 34 to effect the setting of a film advance trigger 35. The positive output from the film advance trigger turns on the camera motor M to initiate a film advancing cycle. The camera gear train drives a cam 36 which coacts with the center strap of transfer camera contacts 37. The normally closed side of the camera contacts is connected to a negative voltage terminal 38 whereas the normally open side is connected to a positive voltage terminal 39 and approximately 70 milliseconds after the camera receives the impulse to feed film, the contacts will transfer to close the positive side and will remain there during film feeding. After one frame of film has been fed, the cam 36 will transfer the camera contacts back to close the negative side. The film advance trigger 35 is reset upon closure of the negative'side of the camera contacts at the end of each feed cycle and similarly, a read trigger 40 is set and the positive output from theread trigger is used to signal the tape control unit 11 that the film feed has stopped and that another selection of tape may now be scanned by the tape reader 12. The read trigger is reset by the end-of-record signal from the tape control unit indicating that a film advance cycle is about to take place, during which time there should be no scanning of the tape.

There has been briefly described thus far the general operation of the printer system. The description to follow will be concerned with the novel limit checking arrangement as applied to a system of the foregoing type.

As was previously mentioned, the primary function of limit check is to check all selection and positioning circuitry of the printer once each frame. It is accomplished in four steps during each film advance cycle as follows:

(a) A blank code square is unblanked approximately 725 lines below the right edge of the normal print area on the screen.

(b) Approximately 453 microseconds later a blank code square is unblanked approximately 7.25 lines above the upper left edge of the normal print area on the screen.

(c) Approximately 453 microseconds after (b) a retrieval code square is unblanked in the same lower right position as in (a).

(d) Approximately 453 microseconds after (c) aretrieval code square is unblanked in the same upper left position as in (b).

Referring to FIGS. la and lb, during a film advance, the camera contacts 37 will transfer and a positive pulse will be directed from the positive terminal 39 over line 41 to an inverter 42 (FIG. 1b) and a 10 microsecond single shot multivibrator 43 to start the limit check operation. The single shot output is transmitted by line 44 to the set side of a clock start trigger 45 and the output line 46 from the 0 or set side of this trigger is a reset line which connects to the reset side of the last stage T9 of a closed clock ring 47 through an AND switch 48. The clock ring 47, when in a reset condition, has stage Tl turned on with an output being delivered from the output terminal 49 of stage T1. The setting of clock trigger 45 serves to disable the clock reset circuit thereby conditioning the ring for operation. The reset line 46 is also connected to the reset side of a binary trigger 50 and thus the binary trigger is also conditioned for operation. A 333 kc. crystal controlled oscillator 51 is stepped down to 167 kc. by the binary trigger 50, the output of which now drives the 9 stage clock ring 6 microseconds per stage to deliver timed outputs T1 through T9 at the output terminals to control the timing of the printer. As shown in FIG. 3, each printer clock cycle is 54 microseconds with the printer going through 36 clock cycles during the total limit check operation.

The limit check start line 44 is also connected to the set side of an LC'l trigger 52 to set this trigger simultaneously with the starting of the clock. Triggers 53, 54 and 55 along with trigger 51 are arranged as an open ring and serve to provide timing controls for the previously mentioned four steps which make up the limit check operation. The open ring triggers may be originally placed in a reset condition by closure of an initial reset switch 56 which connects the reset side of the triggers to a positive potential terminal 57. This open timer ring along with the closed clock ring 47 form the nucleus of the timing control circuitry depicted by box 14 on FIG. 1a. The setting of LCl trigger 52 raises the potential of output line L1 and through the OR switches 58 and 59, the control lines Ll+L2 and Ll+L3 are also raised in potential. The rise on line L1+L2 is passed through an OR switch 60 to an inverter 61 and the resulting inverter negative output is transmitted over line 62 to an AND switch 63 which in turn is connected to the reset side of the clock trigger 45. As a result, the clock trigger 45 is prevented from being reset at T3 time 'of each clock cycle until after the limit check operation has been completed.

In the beginning of the limit check operation, the single shot multivibrator 43 was turned on and at that time, the multivibrator output was also passed through an OR switch 64 and an inverter-shaper 65 to turn on a 400 microsecond single shot multivibrator 66. This multivibrator functions as a limit check delay in that it delays the unblanking of the tube 400 microseconds to allow an interval of time during which the selection and positioning registers are set. The limit check delay output line 67, which has now been raised in potential, is connected to an AND switch 68 and at T9 time of the first clock cycle, switch '68 is activated to effect the resetting of a horizontal advance control trigger 69 and an unblank control trigger 70. Referring to FIGS. 1a and 3, during the first 8- clock cycles,, while the 400 microsecond limit check delay multivibrator 66 is up and the LCl trigger 52 is on to maintain line L1 up, the selection register is reset and the horizontal and positioning registers 16 are set. As shown in FIG. 1a, the timing control line Ll+L2 is connected to the reset side of the selection register 15, thus setting this register at 000000 with the 7th check bit position of the letter code not being used in the limit check operation. The blank code 000000 in the selection register represents the low limit of selection and will result in the selection coils 19 presenting to the aperture 27 a beam of no light intensity as would occur if the beam is correctly aligned with the blank opaque square added to the matrix 24 for limit check operation.

The timing control line L1+L3, which is also up at this time, is connected to the set side of the vertical and horizontal positioning registers 16 and accordingly, these registers are turned on to register the full amount of 111111. This represents the high limit of positioning and will result in the positioning coils taking the beam, when it is unblanked, to the lower right corner of the screen. As shown in FIG. 3, the 400 microsecond limit check delay will fall during the early part of the 8th clock cycle, the single shot '66 flipping and raising the output line 71. Output line 71 is connected to an AND switch 72 and at T7 time of the 8th clock cycle, the unblank control trigger 70 will be set on. With the unblank control trigger output line 73 up and the binary clock trigger output line 74 up, at T4 time of the 9th clock cycle, the AND switch 75 will be activated to set an unblank trigger 76. The unblank control trigger, switch 75 and the unblank trigger, in effect, make up the unbl-ank control box 29 (FIG. 1a), the output of which is passed through the suitable unblank amplifier 30 to cylinder 28 to unblank the aperture 27 and pass the selected beam through for projection on the screen 31. As shown in FIG. 2, 'for the first cycle of operation, the beam representing the blank code is positioned at square 77 in the lower righthand corner of screen 31 and outside of the normal print area 78. The square 77 is shown dotted to indicate that the blank code exposes the screen to the minimum or no amount of light. At T6 time of the 9th clock cycle, the AND switch 79 (FIG. 1b) will set the horizontal advance control trigger 69 and at T1 time of the 10th clock cycle, the unblank trigger 76 will be reset.

Referring to FIG. 1a, a first photo cell 80 is positioned opposite the lower right-hand corner of the screen and a second photo cell 81 is positioned opposite the upper left-hand corner of the screen to sense the unblanked limit check code squares. The output from the photo cells is fed to a low intensity beam checking circult and a high intensity beam checking circuit arranged in parallel. Thelow intensity beam checking circuit comprises an amplifier 82 and one-half of an AND switch 83, the other half of switch 83 being fed from the timing control line L1+L2. The high intensity beam checking circuit comprises a current limiter 84, an amplifier-inverter 85 and one-half of an AND switch 86, the other half of switch 86 being fed from the timing control line L3 +L4. From the switches 83, 86, the two checking circuits are merged into an OR switch 87, the

6 output of which is fed to an AND switch 88. The AND switch 88 is also fed from the limit check delay line 67, the output line 89 from the set side of the horizontal advance control trigger 69, and the T7 terminal of clock ring 47. It Will be remembered that the horizontal advance trigger 69 was set at T6 time of the 9th clock cycle and hence, its output line 89 is up. At T1 time of the 10th clock cycle, the AND switch 89a (FIG. 1b) is activated to turn on the 400 microsecond limit check delay single shot 66 and bring up the limit check delay line 67. At T7 time, then, the AND switch 88 is gated to sample the error checking circuit and to either set or not set a limit check interlock trigger 90, depending on the photo cell response. For the first cycle of limit check operation, the timing control line L3 +L4 is down and the high intensity beam checking circuit is inoperative; however, the timing control line L1+L2 is up and if photo cell 80 senses any amount of light at all, indicating that the beam was erroneously positioned, switches 83 and 88 will be activated to set the interlock trigger 90. If the interlock trigger is set, its output line 91 will come up to cause the clock trigger 45 (FIG. 1b) to reset. Trigger 50 will now reset and clock ring 47 will stop at T1 of the next cycle indicating that the system is not operating properly. While the printer is stopped, the camera will continue its feed cycle until the camera contacts 37 transfer to reset the film advance trigger 35 and stop the camera motor M. After the machine is checked and it is desired to resume operation, a manual reset switch 100 (FIG. 1a) may be closed to reset the interlock trigger 90 and a manual start switch (not shown) in the tapecontrol unit 11 is closed to put the system into operation.

Assuming that photo cell 80 senses no light at all, indicating that the beam was correctly positioned, switch 83 will not be activated and the interlock trigger 90 will not be set. Under these conditions, at T8 time of the 10th clock cycle, the AND switch 92 (FIG. 1b) will be activated to turn on an inverter-shaper 93. The inverter-shaper provides an advance pulse for the LCl- LC4 ring triggers and will turn the LCl trigger 52 off and the LC2 trigger 53 on. The LC3 and LC4 triggers 54, 55 will not be afiected at this time because they had no immediate preceding trigger turned on. Turning on of the LC2 trigger 53 initiates the second limit check cycle and the trigger output line L2 is raised which in turn, through the OR switches 58 and 94, raises the timing control lines L1+L2 and L2+L4. At T9 time of the 10th clock cycle, the AND switch 68 becomes activated to efiect the reset of the horizontal advance trigger 69 and the unblank control trigger 70.

Referring to FIG. 1a, the timing control line L1+L2 is connected to the reset side of the selection register 15 and hence, this register retains the blank code 000000 that it had in the first limit check cycle and a beam of No light intensity will again be selected. The timing control line L2+L4 is connected to the reset side of the vertical and horizontal positioning registers and these registers will now switch from 111111 to 000000. The remainder of the second limit check cycle will proceed in the same fashion as described above in connection with the first cycle and the blank code is unblanked as square 95 positioned in the upper left-hand corner of the screen, as shown in FIG. 2. Thus, in the first two limit check cycles, the vertical and horizontal positioning circuits have been tested for their extreme limits of operation and the selection circuits tested for the low limit of operation.

Assuming no error was sensed and the limit check interlock trigger was not set, the switch 92 (FIG. 1b) will cause the ring to advance to LC3 trigger 54, raising output line L3 and, through the OR switches 59 and 96, raising the timing control lines L1+L3 and L3+L4. The timing control line L3+L4 is connected to the set side of the selection register 15 and hence, this register will be switched to the retrieval code of 111111. The retrieval code 111111 represents the high limit of selection and will result in the selection coils 19 presenting to the aperture 27 a beam of full light intensity as would occur if the beam is correctly aligned with a transparent square added to the matrix 24 for limit check operation. The timing control line L1+L3 is connected to the set side of the vertical and horizontal positioning registers 16 and accordingly, these registers are turned on to register the full amount of 111111. When the full intensity beam is unblanked, it will be positioned as square 97 (FIG. 2) in the lower right-hand corner of the screen where it will be sensed by the photo cell 80. Assuming that the beam was correctly positioned, the output from photo cell 80 will be of a value such that the output from the amplifier-inverter 85 (FIG. 1a) will cancel the signal on line L3 +L4 and the switch 86 will not be activated. The limiter 84 functions to clip the signal to establish a reference level. If the beam had been incorrectly positioned, the output from photo cell 80 would not be sufficient to effect cancellation of the L3+L4 signal and switch 86 would be activated to cause setting of the interlock trigger 90.

For the fourth and last cycle of the limit check operation, the ring is advanced to LC4, raising the output line L4 which in turn, through the OR switches 94, 96, raises the timing control lines L2+L4 and L3+L4. Accordingly, the selection register will again register 111111, the vertical and horizontal registers will be reset to 000000, and the full beam will be unblankcd as square 98 in the upper left-hand corner of the screen and opposite the photo cell 81. In the third and fourth limit check cycles, then, the vertical and horizontal positioning circuits are again tested for their extreme limits of operation and the selection circuits tested for the high limit of operation. The unblank circuits are also checked, since if the beam fails to unblank, the output of photo cell 81 will be insufi'icient to cancel line L3 +L4 and the interlock trigger 90 would be turnedon.

If no error occurred during the fourth limit check cycle, the next advance pulse from the switch 92 (FIG. 1b) will turn off the LC4 trigger and the fall of output line L4 is transmitted by wire 99 to effect the reset of the clock trigger 45. The clock trigger, in turn, through output line 46, will now cause the trigger 50 to reset and the clock ring 47 to stop at T1 of the next cycle. Also, the camera contact 37 now transfers to the negative side to disable the limit check circuitry and to cause the read trigger 40 to set and the film advance trigger 35 to reset. Accordingly, the camera motor M stops so that no more film is advanced and the read trigger 40 signals the tape control unit 11 to start the next tape advance and read cycle.

While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. In a cyclically operable printer system of the class described, a cathode ray tube, cyclically operable means for supplying input data to said tube, selection circuitry and positioning circuitry associated with said tube and controlled by said input data to cause said tube to cyclically display data information, a cyclically operable recording camera for recording the displayed information onto film, said camera including means for advancing a frame of film after a plurality of displays have been recorded, switching means controlled by said film advancing means, and circuit means controlled by said switching means for automatically test operating said selection and positioning circuitry during each film advancing cycle of said camera.

2. A cyclically operable cathode ray tube printer system for transforming digital input information into a character display on the face of the tube comprising, se-

lection circuitry and associated digital-to-analog conver- 'sion circuitry controlled by said input information to select a character for display, positioning circuitry and associated digital-to-analog conversion circuitry controlled by said input information to position the selected character, a control circuit for unblanking the selected character on the face of the tube, a cyclically operable camera for photographing the character display onto film, said camera including means for advancing a frame of film after a plurality of character displays have been photographed, switching means controlled by said film advancing means, and circuit means called into operation by said switching means for automatically test operating said selection, positioning, digital-to-analog, and unblank circuits during each film advancing cycle of said camera to ascertain if the printer system is functioning properly.

3. A cyclically operable cathode ray tube printer system as in claim 2 wherein, said test operate circuit means includes means for exercising the digital-in-analog, selection, and positioning circuits to the minimum and maximum limits of their operation, and a checking circuit operable to check said digital-to-analog, selection, and positioning circuits to determine if they function properly at the afore-mentioned limits of operation.

4. A cyclically operable cathode ray tube printer system for transforming digital input information into a character display on the screen of the tube comprising, selection circuitry and associated digital-to-analog conversion circuitry controlled by said input information to select a character for display, positioning circuitry and associated digital-to-analog conversion circuitry controlled by said input information to position the selected character, a control circuit for unblanking the selected character on the screen of the tube, a cyclically operable camera for photographing the character display onto film, said camera including means for advancing a frame of film after a plurality of character displays have been photographed, switching means controlled by said film advancing means, circuit means called into operation by said switching means for automatically operating said selection, positioning digital-to-analog, and unblank circuits during each film advancing cycle of said camera to unblank in each of two diametrically opposing corners of the screen two characters having complementary codes, and photocell means for sensing said unblanked complementary codes to indicate if the printer system is functioning properly.

5. A cyclically operable cathods ray tube printer system for transforming digital input information into a character display on the screen of the tube comprising, a selection register and associated digital-to-analog conversion circuitry controlled by said input information to select a character for display, horizontal and vertical positioning registers and associated digital-to-analog conversion circuitry controlled by said input information to position the selected character, a control circuit for unblanking the selected character on the screen of the tube, a cyclically operable camera for photographing the character display onto film, said camera including means for advancing a frame of film after a plurality of character displays have been photographed, switching means controlled by said film advancing means, circuit means called into operation by said switching means for automatically setting and resetting all positions of said selection register and horizontal and vertical positioning registers twice during each film advancing cycle of said camera to select and position two characters having complementary codes, circuit means effective during each film advancing cycle of said camera for operating said unblank control circuit whereby there is unblanked in each of two diametrically opposing corners of the screen said two complementary coded characters, and photocell means for sensing said unblanked complementary codes to determine if the printer system is functioning properly.

6. In a cyclicaily operable printer system having a cathode ray tube, a light beam source, a character matrix, and associated control circuitry for transforming digital input information into a character display on the screen of the tube, the combination of, a cyclically operable camera for photographing the character display onto film, said camera including means for advancing a frame of film after a plurality of character displays have been photographed, cam operated switch means controlled by said film advancing means, circuit means called into operation by said switch means for automatically operating the control circuitry of said tube during each film advancing cycle of said camera to unblank in each of two diametrically opposing corners of the screen two characters having complementary codes, one of said characters representing a beam of maximum light intensity and the other of said characters representing a beam of minimum light intensity, photocell means for sensing said two unblanked characters in each of said opposing corners, and interlock circuit means responsive to said photocell means for indicating the absence of either of said complementary coded characters.

7. in a cyclically operable printer system having a cathode ray tube, a light beam source, a character matrix, and associated control circuitry for transforming digital input information into a character display on the screen 7 of the tube, the combination of, a tape reader and tape control unit for supplying said digital input information, a cyclically operable camera for photographing the character display onto film, said camera including means for advancing a frame of film, means controlled by said tape control unit for operating said film advancing means to make a film advancing cycle and feed a frame of film after a plurality of character displays have been photographed, cam operated switch means in said camera and operable by said film advancing means during a film advancing cycle, circuit means called into operation by said switch means to test operate said printer control circuitry during said film advancing cycle, and means controlled by said switch means at the end of la film advance cycle for signaling said tape control unit to supply digital input information to said tube control circuitry.

8. In a cyclically operable printer system having a cathode ray tube, a light beam source, a character matrix, associated selection and positioning registers and an un blank control circuit for transforming digital input information into a character display on the screen on the tube, the combination of, a tape reader and tape control unit for supplying said digital input information, a cyclically operable camera for photographing the character display onto film, said camera including means for advancing a frame of film, means controlled by said tape control unit for operating said film advancing means to make a film advancing cycle and feed a frame of film after a plurality of character displays have been photographed, a cam operated by said film advancing means, switch means in said camera and operable by said cam during a film ad- Vancing cycle, limit check circuit means called into operation by said switch means for automatically developing during said film advance cycle a series of signals which successively set and reset said selection and positioning registers and operate said unblank control circuit to un-blank in each of two diametrically opposing corners of the screen two characters having complementary codes, photocell means for sensing said two unblanked characters in each of said opposing corners, said photocell means developing an output only when the unblanked characters sensed in said opposing corners do not represent complementary codes, and interlock circuit means respon ive to an output from said photocell means for stopping operation of said printer system.

9. A cyclically operable printer system as in claim 8 including means eilective upon the absence of an output from said photocell means for disabling only said limit check circuit means.

10. In a cyclically operable printer system having a cathode ray tube, a light beam source, a character matrix, and associated control circuitry for transforming digital input information into a character display on the screen of the tube, the combination of, a tape reader and tape control unit operable to read digital data and end-ofrecord marks on a tape and to initiate a read cycle during which digital input information is supplied to said control circuitry, a cyclically operable camera for photographing the character display onto film, said camera including means for advancing a frame of film, a read control device responsive to a tape end-of-record mark to terminate said read cycle, an end-of-record indicator device operable in response to an end-of-record mark, a film advance control device responsive to said end-ofrecord indicator device for operating sm'd film advancing means to make a film advancing cycle and feed a frame of film, I3. cam operated by said film advancing means, switch means in said camera and operable by said cam during a film advancing cycle, and circuit means called into operation by said switch means to test operate said printer control circuitry during said film advancing cycle, said film advance control device and read control device being controlled by said switch means at the end of a film advance cycle to disable said film advancing means and to initiate a read cycle.

References Cited in the file of this patent UNITED STATES PATENTS 2,624,798 Dinga Jan. 6, 1953 2,659,828 Elliott Nov. 17, 1953 2,808,768 Squassoni Oct. 8, 1957 2,818,466 Larson Dec. 31, 1957 

